1. Field of the Invention
The present invention relates to radio transmitter systems. More particularly, the present invention relates to Root-Mean-Square (RMS) to DC converters, also called “RMS power detectors.”
2. Discussion of the Related Art
RMS power detectors typically operate within a dynamic range at a wide range of radio frequencies, thus RMS power detectors are found in numerous kinds of communication and instrumentation equipment, with special application in accurate measurements of average power levels of signals over time, independently of signal composition or “wave shape”.
One design challenge for RMS power detectors is to provide a sufficient dynamic range at high input frequencies, as measured by, for example, a ratio of the largest input power level to the smallest input power level that can be accurately detected. Typically, the useable dynamic range of an RMS-DC converter decreases as the input frequency increases. This tradeoff is generally determined by the “square cell”, which is a circuit block that calculates the square of its input signal (i.e. the product of the input signal with itself). Many different square cell structures are known in the literature (see, e.g., U.S. Pat. Nos. 7,622,981, 5,489,868, 5,581,211, 6,549,057 and 7,342,431; see, also, the article “A 2 GHz mean-square power detector with integrated offset chopper” (“Kouwenhoven”) by M. Kouwenhoven et al., published in Digest of Technical Papers, ISSCC 2005, pp. 124-125, February 2005). Although most square cells have a high bandwidth, most have a poor dynamic range because of DC offsets resulting from device mismatches. To achieve a useful dynamic range, many RMS-DC converters are enhanced by dynamic range enhancement techniques. However, such dynamic range enhancement techniques often significantly reduce the maximum operating frequency of the RMS power detector. The highest frequency achieved by RMS power detectors currently available on the market is typically not greater than 10 GHz, with a useable dynamic range of about 30 dB at that frequency. The maximum operating frequency does not scale well with the transition or cut-off frequency (fT) of a bipolar device, which is used in practically all RF RMS power detectors. The transit time of minority carriers through the base does not determine the bandwidth of the square cell. Specifically, the square cell bandwidth is determined by the base resistance and base-emitter junction capacitance.
Most square cell operations are based on the exponential relation in a bipolar transistor between the collector current and the base-emitter voltage. Alternatively, in the few MOS1-based examples, square cell operations may be based on the quadratic relation between the drain current and the gate-source voltage in an MOS device. 1 Metal-Oxide-Semiconductor
The dynamic range of an RMS power detector is primarily limited by the DC offset resulting from mismatches between the bipolar or MOS devices used in the RMS power detector. The bandwidth of an RMS power detector is generally large, limited by junction capacitances, together with the impedance of the voltage source driving the cell. Other methods for realizing a squaring or root-mean-square function include using an analog multiplier as the square cell (see, e.g., Kouwenhoven) or using trans-linear circuit techniques, such as those disclosed in U.S. Pat. No. 7,002,394. The bandwidth of a circuit using any of these techniques is generally lesser than those of other types of square cells, while its dynamic range is similar. Enlarging the devices, though a proven technique to reduce the effects of device offsets, is not effective in enhancing the dynamic range of a square cell. For example, even doubling the device sizes, the resulting offset may be reduced by merely a factor of √2, and thereby improves the output dynamic range by only 3 dB, while the bandwidth is reduced by a factor 2. As a result, due to the quadratic nature of the square cell, the input dynamic range is improved only by 1.5 dB. A dynamic range extension by 9 dB would require the devices to be scaled up by a factor of 64, thereby reducing the bandwidth significantly.
Trimming is another alternative approach to reduce offsets without increasing device sizes. In a square cell, trimming allows a higher bandwidth for a given dynamic range. Trimming, however, cannot remove all offset components from the square cell output signal (see, e.g., Kouwenhoven). Especially over the full operating temperature range the effectiveness of trimming is limited. Large device sizes are required to suppress the remaining offset components.
The prior art relies significantly on increasing device sizes to improve the square-cell dynamic range, at the expense of a reduced bandwidth of the square cell. However, device scaling cannot achieve both high bandwidth and a sufficiently wide dynamic range. Numerous circuit techniques may be used to increase the dynamic range of a square cell that is suitable for use in a high dynamic range RMS-DC converter, although most of these techniques limit the attainable RMS detector bandwidth.
As the dynamic range of an output signal of a square cell is much greater than the dynamic range of its input signal (typically at least two times in dB), both small offsets and noise components in the output signal path can significantly reduce the overall dynamic range of an RMS power detector. One method to increase the dynamic range of an RMS power detector is to compress the dynamic range of the square cell output signal, using a circuit with a non-linear transfer function. For example, U.S. Pat. No. 7,342,431 discloses a square cell followed by a DC LOG-amp. Such an approach has the advantage that only the square cell operates at high frequencies, while the rest of the circuitry is DC, thus enabling high-frequency operation, in principle. The disadvantages of such an approach includes (a) the effect of an offset generated inside the square cell on the dynamic range of the RMS power detector is not mitigated (i.e., to achieve a reasonable dynamic range, large devices and offset trimming are still required); (b) the attainable bandwidth is still significantly limited by the required large device sizes; and (c) temperature drift and frequency dependence of the square cell transfer function are directly observable in the overall detector transfer function.
Other RMS-DC converters extend the dynamic range of an RMS power detector by placing a variable gain amplifier (VGA) at the input terminal of a square cell, as depicted in FIG. 1. As shown in FIG. 1, input signal 101 is provided to VGA 102, which gain is controlled by VGA gain control circuit 105 based on output signal 104, in such a way that the magnitude of input signal 108 of square cell 107 is kept substantially constant over the full input power dynamic range of RMS power detector 100. Low pass filter 106 removes the unwanted high frequency components from the output signal of the square cell and RMS power detector 100. The impact of any offset voltage in square cell 107 is significantly reduced, since output signal 104 is much larger than the offset over the full detector dynamic range.
RMS power detector 100 has the advantage that it allows realization of a wide dynamic range RMS-DC converter with high accuracy and temperature stability. RMS power detector also has an disadvantage that, as VGA 102 processes RF input signal 101, VGA 102 requires a large bandwidth over the full VGA gain range (e.g., at least equal to the required detector dynamic range in dB). Another disadvantage of RMS power detector 100 is that its attainable bandwidth is primarily limited by the bandwidth of the VGA. The maximum operating frequency of RMS power detector 100 is thus associated with the bandwidth of the VGA, which in turn increases with the bipolar transition frequency fT.
Another approach to extend dynamic range uses low-frequency feedback in combination with two square cells, as depicted in FIG. 2. As shown in FIG. 2, first square cell 207a is driven by high-frequency input signal 201, while second square cell 207b is driven by a DC signal, which is derived by feedback control circuit 209 from detector output signal 204. Feedback control circuit 209 has a (non-linear) transfer function that determines the overall transfer of the power detector. At sufficiently high loop gain, the feedback loop equalizes the output signals 205a and 205b of square cells 207a and 207b, respectively. The difference 206 of output signals 205a and 205b is integrated by integrator 208 to provide output signal 204 of RMS power detector 200. For a high loop gain, the overall transfer of the detector—from RF input power to DC output voltage—becomes equal to the inverse of feedback transfer 209.
RMS power detector 200 has similar characteristics to the RMS power detector described above that compresses the dynamic range of the square cell output signal using a circuit with a non-linear transfer function. Again, only a single square cell need to operate at high frequencies, thus enabling a high RMS power detector bandwidth to be achieved. Another advantage is the matching of temperature drift and device offsets, so as to improve the accuracy and dynamic range. These advantages are compared to those achieved for the RMS power detector that compresses the dynamic range. The disadvantages of RMS power detector 200 includes (a) requiring both large device sizes and trimming of the square cells to achieve a reasonable dynamic range, thereby undesirably reducing the attainable bandwidth; (b) the feedback loop typically results in a slow detector response (i.e., a lengthy response time), and (c) potential instability since the loop gain varies significantly as a function of the input power level applied to the detector.
Yet another bandwidth extension technique uses linear analog multipliers to carry out the squaring operations, and applies chopping to eliminate device offsets generated inside the multipliers. This technique is discussed in Kouwenhoven and U.S. Pat. No. 7,197,292. The analog multipliers can implement chopping, which is a much more effective way for reducing offsets in a square cell than both device scaling and trimming, and potentially allows higher detector operating frequencies. A multiplier-based square cell, however, has a considerably lower bandwidth than other square cells and is therefore less suited to realize an RMS-DC converter at very high frequencies. Also, a multiplier-based square cell operates at much higher current densities and results in higher overall power consumption.